1. Field of the Invention
The invention, as indicated in the title, refers to the construction of segmented lattice racks usable in nuclear power stations for the storage of fuel, both fresh and irradiated, in a swimming-pool full of water or boron treated water which acts as a cooling and radiological shielding medium. The rack has neutronic poisons to prevent criticity. The fuel components are prismatic elements with a height much greater than the other dimensions.
2. Description of Related Art
In countries with nuclear programs in development and nuclear power stations under construction, in the area of irradiated fuel there was a need to densify the racks of the storage pools, which nuclear power stations have for such a purpose, by using stainless steel racks, equal to those available in the past but bringing the fuels closer together, permitting storage capacity to be increased. At that time, at the end of the seventies and beginning of the eighties, the racks had stainless steel channels without any additional material acting as a neutronic poison since the distance or passage between the cells was great enough so that the water, whether treated with boron or not, existing between cells (xe2x80x9cwater gapxe2x80x9d) was sufficient to maintain the criticity at acceptable levels. They had a lower mesh for the support of the fuel components and were anchored both to the bottom and sides of the pool to form a single interlocked assembly, with effective performance even during earthquakes.
As a result of this approach of the stored fuel components and to increase the capacity of the pools, it was necessary to use an additional material to prevent criticity. The materials used more frequently as neutron poisons are, apart from water itself, whether treated with boron or not, boron treated steel and Boral (a dispersion of a boron carbide in aluminium).
Currently, two types of rack construction exist: one consisting of constructing the vertical channels of the entire height of the fuel component from welded pipes and plates forming pipes and joining the channels to form the rack, and another one consisting of forming the rack by means of different levels of interlaced or not-interlaced lattices until completing the necessary height. Racks of this type are formed throughout their height (longitudinally) by several piled lattices.
German Patent DE-2930237 foresees a device of these features in which each one of the lattices consists of plate strips which cross, being superimposed and packed as a whole by some enveloping steel bands forming a complex structure. The coupling of the different plates forming the lattices between each other is performed across grooves on both faces thereof, being orthogonally joined through this area in a known dovetailed solution.
European Patent Applications EP-0759623 and EP-0769785, of CCI AG, refer to an assembly for nuclear fuel storage and to a storage rack for said fuel in which there are also several lattice units whose plates in one direction are at the same level as the perpendicular ones, but in their upper and lower parts have some outgoing and incoming shapes, related to each other (one lattice opposite the other) which are coupled in the pile, preventing lateral sliding in any direction.
U.S. Pat. No. 5,032,348 is designed for dry storage and transportation of dry fuel, not in swimming pools as in the present invention, and basically provides a sandwich of structural steel cylinders with an inner framework. The structural function is performed by the outer steel cylinders and not by the framework.
French Patent FR 2,433,226 refers to a framework designed for storing nuclear fuel, forming a double wall interlocked matrix.
The rack proposed in the present invention shares, like all those of this type existing until now, the idea of forming lattices which are piled forming a sandwich in which a plurality of bottle holder type vertical cavities have been defined and in which each one of the irradiated fuel components coming from the power stations is stored. Similar to the documents initially mentioned, these lattices are formed by means of lines of plates grooved on both faces, over which are coupled, at a different level, another line of plates perpendicular to the former, all of them dovetailed to each other across the opposite grooves arranged for such a purpose.
Different to previous documents, in the rack of the this lattice structure, the plates forming this lattice structure, coinciding with the active part of the radioactive component, consists of a material formed from neutronic poisons, preferably boron treated steel, consisting of a single plate or in a sandwich consisting of two boron treated steel plates with an intermediate gap to permit the passage of water, whether boron treated or not, from the pool. Also a sandwich formed by two stainless steel plates separated by Boral dispersion of a boron carbide in aluminium may be used. The plates forming the end zones, coinciding with the non-active part of the stored radioactive component are of normal stainless steel. These last plates are securely welded to each other. With this height difference of materials, a saving thereof is permitted since only special materials are employed, expensive ones, in the required areas. Besides, the manufacture is with few or no welds in the central body and hence with a significant reduction of assembly inspection and manufacturing costs.
To the rack constructed in this way, another with similar characteristics may be coupled above, having equal or less height, to pile the fuel components at two levels.
The plates constituting each one of the intermediate lattices, with that situated in the same upper and/or lower plane, besides being situated at 90xc2x0 rotation to fit into each other, preventing lateral movement in any direction, may be joined by means of angle or xe2x80x9cTxe2x80x9d welding, to form a rigid assembly if the mechanical design conditions so require. But in most cases in which the mechanical design conditions do not so demand, or in those countries where the legislation impedes welding to be performed in this area or where the materials integrating these plates are not weldable, a vertical tying of the different lattices is performed by means of pretensioned thin strips welded on the lower and upper stainless steel lattices which are packed together once the structure is assembled and hence preventing intermediate welds.